Mac or Windows? Exact version of Photoshop CS6 (example Photoshop 13.0.6 for the Mac). Exact version of ACR and of your OS?
Please read this FAQ for advice on how to ask your questions correctly for quicker and better answers:
The D600 has been supported since ACR 7.3. Try installing ACR 8.3 for CS6 (not the 8.3 version for CC, careful!), or even the latest beta, ACR 8.4RC (Release Candidate, which means advanced beta).
Do you mean updated to the 8.4 RC plug-in?
If so, do note that it is not compatible with older OS versions:
"This release candidate has dropped support for Windows XP, Windows Vista, and Mac OSX 10.6 for Photoshop CS6 (and CC) customers."
If you need help getting back to ACR 8.3, let me know.